interactions of fungi from herbivore dung (see Fig. 10.8)
and it helps to explain how the Basidiomycota can
ultimately dominate a fungal community, truncating
the activities of other fungi (Ikediugwu 1976). It also
explains why the Basidiomycota tend to be mutually
exclusive. For example, toadstools of both Coprinus
heptemerus and Bolbitius vitellinusoccur on dung, but
never in the same piece of dung because these two
species antagonize one another (Ikediugwu & Webster
1970).
Hyphal interference occurs rapidly, a few minutes
after hyphal contact, and often is localized to a single
hyphal compartment. The first visible sign is vacuola-
tion and loss of turgor in the affected hyphal com-
partment. If a dye such as neutral red is added at this
stage it is taken into the affected compartment but does
not enter the healthy hyphal compartments, indicat-
ing that hyphal interference causes loss of normal
membrane integrity. In electron micrographs the
affected cytoplasm is seen to be degenerate, the mito-
chondria are swollen, and a wide gap is seen between
the retracted plasma membrane and the hyphal wall.
The damage is often contained within a hyphal com-
partment by zones of dense, coagulated cytoplasm on
either side of the contact point. The mechanism of
hyphal interference is still unclear, but a poorly diffusible
factor seems to be involved, because hyphal interfer-
ence can occur between hyphae separated by a cello-
phane membrane up to 50μm wide.
Hyphal interference is not a parasitic phenomenon,
because there is no evidence of invasion of the
damaged hyphae. Instead, it seems to be an efficient
means of inactivating other hyphae that are potential
competitors for the same substrates.
Control of pine root rot by hyphal
interference
Soon after hyphal interference was discovered in
the 1970s, it was recognized as a possible basis for
explaining an important biological control system –
the control of pine root rot by an antagonistic fungus,
Phlebiopsis gigantea. Pine root rot is caused by
Heterobasidion annosum(Basidiomycota), which is the
most important pathogen of coniferous trees in the
northern hemisphere, responsible for major economic
losses in the forestry industry. This fungus grows
slowly but progressively along the woody roots, rotting
them and spreading from tree to tree by root-to-root
contact. Eventually, it can spread into the base of
the trunk, causing a butt rotwhich destroys some of
the most valuable timber. When established in a site,
Heterobasidionis almost impossible to eradicate, except
by mechanical extraction of all the infected stumps and
major roots. So, control measures have focused on
preventing it from becoming established, especially in
newly afforested sites (Fig. 12.8).
Heterobasidionproduces air-borne basidiospores from
bracket-shaped fruitbodies at the bases of infected
trees. These spores pose little threat in undisturbed
forests because they have insufficient food reserves to
initiate infection of woody roots when washed into the
root zone. However, the situation is different in com-
mercial forestry, where trees are felled for harvest or
thinned to create the desired plant density as the
plantation develops. The tissues of the exposed stump
surfaces can remain alive for several months, but
with declining resistance to infection. These exposed
stumps provide a highly selective environment for242 CHAPTER 12
Fig. 12.7Hyphal interference on an
agar plate. Hyphae of Heterobasidion
annosum (previously termed Fomes
annosus) have been antagonized
where they were contacted by single
hyphae of Phlebiopsis gigantea(previ-
ously termed Peniophora gigantea).
The agar plate was then flooded with
a dilute solution of neutral red, which
was not taken up by the undamaged
hyphae but entered the damaged
hyphae of Heterobasidion (seen as
darker pigmentation).